1. Field of the Invention
The present invention relates to etchable conductive foils useful in preparing printed circuit boards and particularly to a means and a method for protecting such foils from damage during the handling and further processing of the foil leading up to the etching procedures whereby the foil is converted into the lines and other components making up the conductive pathways for printed circuit board circuitry.
2. Description of the Prior Art
Printed circuit components have become widely used in a variety of applications for radios, televisions, computers, etc. Of particular interest are multi-layer printed circuit board laminates which have been developed to meet the demand for miniaturization of electronic components and the need for printed circuit boards having a high density of electrical interconnections and circuitry. In the manufacture of printed circuit boards, raw materials, including conductive foils, which are usually copper foils, and dielectric supports comprising organic resins and suitable reinforcements, are packaged together and processed under temperature and pressure conditions to produce products known as laminates. The laminates are then used in the manufacture of printed circuit board components. In this endeavor the laminates are processed by etching away portions of the conductive foil from the laminate surface to leave a distinct pattern of conductive lines and formed elements on the surface of the etched laminate. Further laminates and/or laminate materials may then be packaged together with the etched product to form a multi-layer circuit board package. Additional processing, such as hole drilling and component attaching, will eventually complete the printed circuit board product. As printed circuit board technology has advanced to provide higher density boards with smaller printed circuit lines, surface contamination of the raw material products has become a significant problem which generally cannot be tolerated in a successful commercial application.
Although many of the printed circuit board manufacturing and handling procedures are potential sources of surface contamination, one significant problem area involves the construction and lamination procedures by which the laminated products are prepared. Generally speaking, in this connection, the construction of the laminates involves stacking (or lay-up) of conductive foil pieces and dielectric substrates (prepregs) which will then be bonded to one another in a laminating press. Residual particles, particularly those emanating from the prepreg raw materials, are known to be present in the lay-up room environment as a result of the handling of such materials. Eventually these particles may contaminate the surfaces of the conductive foil pieces bonded to the laminates.
Presently such contamination problems are dealt with by attempting to maintain a clean environment in the lay-up area by air filtration, intensified housekeeping techniques, etc. Additionally, the conductive foil surfaces of laminates may often simply be spot cleaned. However, spot cleaning is not necessarily an acceptable procedure for laminates clad with so-called double-treated, electrodeposited copper foils. Double-treated copper foils are those which have been treated on both the matte side and the shiny (drum) side for enhanced bonding ability and improved peel strength when bound to a prepreg. In this regard, it should be noted that while double-treated copper foils are theoretically more desirable than single-treated foils for preparing multi-layer laminates, because such prior treatment enables the elimination of the chemical oxide roughening step, i.e., black oxide, that is otherwise required to enhance bonding and peel strength of the shiny side of the lines after etching, double-treated copper foils cannot be subjected to spot cleaning for removal of contamination because such cleaning would also remove the treatment and prevent, or at least interfere with, subsequent bonding of the conductive lines to another prepreg during fabrication of a multiple layer laminate.
Thus, in the past, surface contamination has been a major economical disadvantage in the production of printed circuit boards, and particularly in the production of multiple layer printed circuit board products having high component densities. Moreover, even through the use of such measures as the environmental control of the lay-up room area, those involved in the manufacture of printed circuit boards have not been able to achieve acceptable prevention of surface contamination without incurring major costs and losses of efficiency in manufacturing procedures. Spot cleaning of laminate surfaces is simply expensive and inefficient, and as indicated above, cannot be tolerated in applications which employ double-treated foils. Accordingly, until the advent of the present invention, surface contamination has remained a significant and substantial problem to the printed circuit board industry.